There's a high end GPU taping out now for release H1 2020, whether it's AMD or NVidia is unknown atm(At least to me) but it'd make sense if that was RDNA2's debut I think (I mean, they can't have consoles launch with a high end version of RDNA2 before PC surely, and they wouldn't bring out 2 enthusiast-grade cards within 10 months surely).
Rumours of an earlier big GPU are probably just a left over from the huge list of things AdoredTV got wrong about Navi.